In the present technical field, LED flip chip die-bond conductive connection generally adopts two manners, that is, low-temperature tin-gold alloy eutectic conduction and ACA (Anisotropic Conductive Adhesive) conductive adhesion, while the ACA is classified into ACF (Anisotropic Conductive Films) and ACP (Anisotropic Conductive Pastes) according to different colloidal states. A main difference between the ACF and the ACP is as follows: the ACF has anisotropic conductivity before process implementation, while the ACP can reflect the anisotropic conductivity by virtue of certain process flows. A technology closest to the present embodiment is the ACP.
Simply speaking, the ACP is non-conductive in X-axis and Y-axis directions and conductive on a Z axis. A conductive principle of the ACP is as follows: “conductive particles and insulating particles (mean particle size of the conductive particles is greater than that of the insulating particles) are added into a synthetic resin adhesive for uniformly mixing, the adhesive is respectively coated on surfaces of adhered conductors and pressurized and heated, and then adhesive liquid flows, so that a distance between the two conductors is close to a diameter of the conductive particles, and the adhesive is cured. The cured adhesive layer is conducted in a vertical direction (a pressurization direction) due to mutual contact between the conductive particles and the bonded conductors, and insulated in a horizontal direction due to actions of the insulating particles.” [1]. A doping ratio of the conductive particles is generally 5%-25% [2].
[1] Research and Application Status of Anisotropic Conductive Adhesives Xiang Hao, Zeng Liming, Hu Chuanqun, Material study and Engineering College of Wuhan University of Technology
[2] Research of Novel Anisotropic Conductive Adhesives Lei Zhihong Polymeric Chemistry and Physics Major ACP conductive particles in Shanghai University:
Conductive fillers are classified into metals, inorganic fillers and mixed fillers. The metals include gold powder, silver powder, copper powder, nickel powder, nickel carbonyl, palladium powder, molybdenum powder, zirconium powder, cobalt powder as well as alloy fillers such as silver-plated metal powder, copper-plated aluminum powder and the like; common inorganic fillers include graphite, carbon black or a mixture of the graphite and the carbon black; and the mixed fillers are products obtained by performing conductive treatment on the metals and the inorganic fillers, such as silver-plated glass beads, silver-plated silica dioxide powder, silver silicides, silicon carbide, tungsten carbide, nickel carbide, palladium carbide, etc.” [1]
ACP Implementation Process [2]:
A process flow for conductive adhesive connection is as follows: performing silk-screen printing on conductive adhesives on a substrate→electrocoating non-conductive adhesives→mounting elements on an upper surface of the substrate→curing the conductive adhesives→detecting. An ACP die-bond mounting and assembling LED flip chip structure is shown in FIG. 1.
Problems in the ACP:
“Conductive fillers are key components of the conductive adhesives and endow the conductive adhesives with conductivity properties, and common conductive fillers include gold, silver, copper, nickel and carbon. In various conductive adhesives, due to high cost, generally gold conductive adhesives are rarely used and are mainly applied to relatively high-end fields such as space industry and the like. Although silver conductive adhesives have been applied to electronic products, because silver molecules easily make electrolytic movements, that is, silver migration, under a direct-current electric field and a humid condition, the conductive adhesives have instable conductivity properties and then are limited in applications. Because the copper powder has active chemical property and is easily oxidized by oxygen and water vapor in air, service life of copper conductive adhesives is shortened. Due to high resistance, carbon black and graphite conductive adhesives are unsuitable to serve as microelectronic packaging materials.” [2]
“ACA novel conductive filling particles are composed of two parts, wherein a particle core refers to epoxy resin particles, and a silverlcopper metal layer is coated outside the core. The novel conductive filling particles also have advantages of silver powder and flexible conductive particles and the like. Densities and thermal expansion coefficients of silver/copper-plated epoxy resin based composite conductive particles are close to those of matrix epoxy resin, sedimentation is not caused even if the conductive particles are used for a long time, and environmental aging resistance of the conductive adhesives can be improved. Plastic deformation of the particles may occur under a pressure effect, thereby enlarging an effective contact area between the conductive particles and electrodes and improving electrical properties of the conductive adhesives.” [2]
However, an existing ACA material still has defects such as low conductivity, poor toughness, instable adhesive effects on different base materials, long curing time and the like. The ACA may be improved in several aspects in future as follows:
(1) Development of a novel base material type: EP serves as the most common base material and has defects of high curing temperature, low storage temperature, poor heat resistance and the like. Development of a novel matrix contributes to widening an application range of the ACA.
(2) Development of a novel conductive filler usage of precious metal ions is reduced, which is beneficial for reducing cost and improving efficiency.
(3) Improvement of thermal stability is an important modification trend.
(4) A dispersion technology of conductive particles: the more uniform the dispersion is, the less the agglomeration is, and the more beneficial to maximization of conductive efficiency the effect is.” [3]
[3] Research Progress on Anisotropic Conductive Adhesives Xu Ruijie, Lei Caihong, Li Shanliang, Huang Weiliang, Material and Energy College of Guangdong University of Technology
“In view of this, a doctor Chen Wencheng in Osram adds that: an existing bottleneck problem of csp is a process problem. If traditional material equipment cannot be directly used at present, even if the csp can reduce cost of products to the greatest degree, production and processing cost is increased. If the csp cannot provide convenience for terminal lighting enterprises, the doctor personally thinks that popularization is very difficult.” [4]
[4] “Focusing on OFweek Leader Round-Table Summit to Perform 360-Degree Understanding on LED Package-Free and Flip Chip Technology”. This article comes from ofweek semiconductor lighting network, Nov. 22, 2014.
Thus, it can be seen that problems in the ACP used for die-bond conductive adhesion are as follows: metal conductive particles must be added into the ACP, and high cost is caused due to a complicated manufacturing process of the metal particles. The metal particles have poor dispersity and low conductivity in colloid and are high in contact resistance. In a process of implementing the die-bond conductive adhesion by the ACP, an operation process is complicated, and processing cost is high; and since the metal particles exist in ACP colloid, the metal particles are easy to precipitate in the colloid.
The above problems inevitably cause problems of high equipment investment, obvious production process problems, low production efficiency, high cost, poor electrical property and product reliability, difficulty in colloid preservation and the like.